Fix mis-synced version check in black.vim (#4567)

The message has been updated to indicate Python 3.9+, but the check
still compares to 3.8

Author: mgorny

CHANGES.md

@@ -20,7 +20,7 @@
 
 ### Packaging
 
-<!-- Changes to how Black is packaged, such as dependency requirements -->
+- Fix the version check in the vim file to reject Python 3.8 (#4567)
 
 ### Parser
 

autoload/black.vim

@@ -75,7 +75,7 @@ def _initialize_black_env(upgrade=False):
       return True
 
   pyver = sys.version_info[:3]
-  if pyver < (3, 8):
+  if pyver < (3, 9):
     print("Sorry, Black requires Python 3.9+ to run.")
     return False